Steel wire rod, steel wire, and part

ABSTRACT

Steel wire rod etc. enabling elimination the heat treatment processes of spheroidizing annealing and of quenching and tempering are provided. A predetermined chemical composition is provided, 90% or more of the microstructure by area ratio is bainite, the mean bainite block size of the surface layer measured at the cross-section is 15 μm or less, the ratio of the mean bainite block size of the surface layer measured at the cross-section and the mean bainite block size measured at the center part is less than 1.0, and the mean particle size of cementite dispersed in bainite is 0.1 μm or less.

FIELD

The present invention relates to steel wire rod, steel wire manufactured by that steel wire rod, and parts manufactured by that steel wire and having a tensile strength of 700 MPa to 1200 MPa. Note that, the “parts” covered by the present invention include machine parts and building parts.

BACKGROUND

Automobiles and various industrial machinery use high strength machine parts having 700 MPa or more tensile strength for the purpose of lightening the weight and reducing the size. In the past, this type of high strength machine part has been manufactured by successively hot rolling and spheroidally annealing a steel material comprised of alloy steel consisting of carbon steel for machine structure use to which Mn, Cr, Mo, B, and other alloy elements are added to soften it, then cold forging or rolling this to obtain a predetermined shape and then quenching and tempering it to impart strength.

However, such a steel material contains large amounts of alloy elements, so the steel material becomes high in cost. Further, it requires spheroidizing annealing before formation of a part and quenching and tempering after forming it, so the manufacturing cost swells.

Due to such a situation, the art of eliminating the spheroidizing annealing and quenching and tempering, performing rapid cooling or aging to raise the strength of the steel wire rod, and then drawing this so as to impart a predetermined strength is known. This art is utilized for machine parts etc. Machine parts etc. manufactured using this art are called “non-heat treated machine parts”.

Japanese Unexamined Patent Publication No. 2-166229 discloses a method of production of a non-heat treated machine part of a bainite structure comprising rolling steel containing C: 0.03 to 0.20%, Si: 0.10% or less, Mn: 0.7 to 2.5%, a total of one or more of V, Nb, and Ti: 0.05 to 0.30%, and B: 0.0005 to 0.0050% to a steel wire rod, then cooling it by a 5° C./sec or more cooling rate.

Further, Japanese Unexamined Patent Publication No. 8-41537 discloses a method of production of a high strength machine part comprising heating steel containing C: 0.05 to 0.20%, Si: 0.01 to 1.0%, Mn: 1.0 to 2.0%, S: 0.015% or less, Al: 0.01 to 0.05%, and V: 0.05 to 0.3% to 900 to 1150° C. in temperature, then hot rolling and finish rolling it, then cooling in a temperature region of 800° C. to 500° C. by a 2° C./sec or more mean cooling rate to thereby obtain a ferrite+bainite structure, then annealing it in a 550 to 700 temperature range.

However, in these methods of production, strict control of the cooling rate and cooling end temperature is required and the methods of production are complicated, so the manufacturing costs swell. Further, the structure becomes uneven and the cold forgeability sometimes deteriorates.

To deal with this, Japanese Unexamined Patent Publication No. 2000-144306 discloses steel for cold forging use containing C: 0.40 to 1.0 mass %, having a chemical composition satisfying a specific condition, and having a structure comprised or pearlite or degenerate pearlite. This steel contains a large amount of C and therefore is inferior in cold forgeability compared with the carbon steel for machine structure use or alloy steel for machine structure use which is used for machine parts in the past.

In the above way, in the non-heat treated steel wire rod according to the prior art, machine parts having excellent cold forgeability and steel wire and steel wire rod for producing those parts could not be obtained by an inexpensive method of production. In particular, in the prior art eliminating spheroidizing annealing, quenching and tempering, etc., the structure becomes uneven and excellent cold forgeability cannot be obtained, so there was room for improvement in the development of parts able to realize excellent mechanical characteristics even if omitting these treatments.

SUMMARY Technical Problem

The present invention was made in consideration of the above problem in the prior art and has as its object the provision of

(a) parts with a tensile strength of 700 to 1200 MPa able to be inexpensively manufactured and (b) steel wire enabling elimination of spheroidizing annealing, quenching and tempering, and bluing after cold forging used for manufacture of those parts and steel wire rod for manufacturing that steel wire.

Solution to Problem

The inventors surveyed the relationship between the chemical composition and structure of steel materials for obtaining high strength parts with a tensile strength of 700 MPa or more for achieving the above object which enables cold forging even if eliminating spheroidizing annealing and even if not performing heat treatment of quenching and tempering. The present invention was made based on the metallurgical findings obtained by such a survey and has as its gist the following:

(1) Steel wire rod comprised of, by mass %, C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, and N: 0.001 to 0.010% and having a balance of Fe and unavoidable impurities, in which steel wire rod, 90% or more of the microstructure by area ratio is bainite, a mean bainite block size of the surface layer measured at the cross section is 15 μm or less, a value of a ratio of the mean bainite block size of the surface layer measured at the cross section and the mean bainite block size measured at the center part (mean bainite block size of surface layer)/(mean bainite block size at center part) is less than 1.0, and a mean particle size of cementite dispersed in bainite is 0.1 μm or less.

(2) The steel wire rod according to (1), wherein the steel wire rod further comprises, by mass %, one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10%.

(3) Steel wire comprised of, by mass %, C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, and N: 0.001 to 0.010% and having a balance of Fe and unavoidable impurities and obtained by drawing, in which steel wire, 90% or more of the microstructure by area ratio is bainite, at the surface layer of the steel wire, a mean aspect ratio R of block particles of bainite measured at the longitudinal section is 1.2 to 2.0, a mean bainite block size of the surface layer measured at the cross section is (15/R) μm or less, a value of a ratio of the mean bainite block size of the surface layer measured at the cross section and the mean bainite block size measured at the center part (mean bainite block size of surface layer)/(mean bainite block size at center part) is less than 1.0, and a mean particle size of cementite dispersed in bainite is 0.1 μm or less.

(4) The steel wire according to (3), wherein the steel wire further comprises, by mass %, one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10%.

(5) The steel wire according to (3) or (4), wherein a critical upsetting rate is 80% or more.

(6) A part comprised of, by mass %, C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, and N: 0.001 to 0.010% and having a balance of Fe and unavoidable impurities, in which part, 90% or more of the microstructure by area ratio is bainite, at a surface layer of the part, a mean aspect ratio R of block particles of bainite measured at the longitudinal section is 1.2 to 2.0, a mean bainite block size of the surface layer measured at the cross section is (15/R) μm or less, a value of a ratio of the mean bainite block size of the surface layer measured at the cross section and the mean bainite block size measured at the center part (mean bainite block size of surface layer)/(mean bainite block size at center part) is less than 1.0, and a mean particle size of cementite dispersed in bainite is 0.1 μm or less.

(7) The part according to (6), wherein the part further comprises, by mass %, one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10%.

Advantageous Effects of Invention

According to the present invention, it is possible to inexpensively provide high strength parts with a tensile strength of 700 to 1200 MPa contributing to the lightening of weight and reduction of size of machine parts used in automobiles and various industrial machinery etc. and building parts used on construction sites.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an SEM photograph showing the microstructure of a steel wire rod and drawn steel wire according to the present embodiment and parts according to the present embodiment.

DESCRIPTION OF EMBODIMENTS

The inventors, as explained above, surveyed in detail the relationship between the chemical composition and structure of steel materials for obtaining high strength parts with a tensile strength of over 700 MPa which enables cold forging even if eliminating spheroidizing annealing and even if not performing heat treatment of quenching and tempering. Further, the inventors proceeded with a comprehensive study of in-line heat treatment utilizing the heat held at the time of hot rolling the steel wire rod and the series of methods of production of the subsequent drawn steel wire and parts based on the metallurgical findings obtained by the survey so as to inexpensively produce high strength parts and reached the following conclusions:

(a) Drawn steel wire made high in strength by drawing and cold forging is inferior in workability, high in deformation resistance, and susceptible to work cracks.

(b) To improve the workability of the high strength drawn steel wire, it is effective to make the structure one mainly comprised of bainite to refine the block particle size of the surface layer and to make the mean particle size of cementite dispersed in the bainite 0.1 μm or less.

(c) That is, by making the area ratio of bainite 90% or more and when making the mean aspect ratio of the block particles of the bainite measured at the longitudinal section a value of R, by making the mean value of the block particle size of the bainite at the surface layer measured by the cross section (15/R) μm or less, and by making the ratio of the mean bainite block size at the surface layer and the mean bainite block size inside the steel wire rod less than 1.0, the cold formability can be remarkably improved.

(d) Furthermore, by making the structure one like in the above (b) and (c), it is possible to raise the yield ratio even if eliminating bluing after forming the part.

In this way, by improving the chemical composition and structure of the steel material, it is possible to increase the strength even if eliminating quenching and tempering and possible to improve the cold forgeability.

Such drawn steel wire forming the material for obtaining a part enabling cold forging even if eliminating the spheroidizing annealing and becoming high strength even without performing the heat treatment of quenching and tempering already has the above characterizing microstructures at the stage of the drawn steel wire. It is effective to work this into a part without performing heat treatment before working.

In this case, compared with the conventional method of production of performing spheroidizing annealing to soften the material, the cold formability deteriorates, but it is possible to cut the cost of spheroidizing annealing and the cost of quenching and tempering after working, so in terms of costs, the present invention is more advantageous.

Furthermore, regarding the method of production of the steel wire rod forming the material of the drawn steel wire, it is possible to utilize the residual heat at the time of hot rolling to immerse the steel wire rod in a molten salt bath immediately after rolling to thereby obtain a steel material of the above-mentioned structure even without adding large amounts of alloy elements.

That is, the parts of the present invention are manufactured by a series of methods of production of immersing a steel material adjusted in chemical composition in a molten salt bath utilizing the residual heat at the time of hot rolling so as to form a mainly bainite steel wire rod of a predetermined mean block size and cementite particle size, drawing this at room temperature under specific conditions to prepare high strength bainite, and forming parts.

For this reason, the present invention enables the inexpensive manufacture of parts with a tensile strength of 700 to 1200 MPa.

Chemical Composition

The chemical composition of the steel wire rod and drawn steel wire for part use with a tensile strength of 700 to 1200 MPa according to the present embodiment (below, sometimes respectively referred to as the “steel wire rod” and “drawn steel wire”) and the parts according to the present embodiment (below, sometimes simply referred to as the “parts”) will be explained. The drawn steel wire according to the present embodiment is obtained by drawing the steel wire rod according to the present embodiment. Further, the part according to the present embodiment is obtained by cold forging the drawn steel wire according to the present embodiment or by cold forging and rolling. The drawing, cold forging, and rolling do not affect the chemical composition of the steel. Therefore, the explanation regarding the chemical composition explained below covers all of steel wire rod, drawn steel wire, and parts. In the following explanation, “%” means “mass %”. Note that, the balance of the chemical composition is Fe and unavoidable impurities.

C: 0.15 to 0.30% C is an element required for securing tensile strength. If the C content is less than 0.15%, it is difficult to obtain a 700 MPa or more tensile strength. Preferably, the C content is 0.20% or more. On the other hand, if the C content is over 0.30%, the cold forgeability deteriorates. Preferably, it is 0.25% or less.

Si: 0.05 to 0.50%

Si is a deoxidizing element and an element increasing the tensile strength by solid solution strengthening. If the Si content is less than 0.05%, the effect of addition is not sufficiently exhibited. Preferably, the Si content is 0.15% or more. On the other hand, if the Si content is over 0.50%, the effect of addition becomes saturated and the ductility at the time of hot rolling deteriorates resulting in defects easily forming. The preferable Si content is 0.30% or less.

Mn: 0.50 to 1.50%

Mn is an element raising the tensile strength of steel. If the Mn content is less than 0.50%, the effect of addition is not sufficiently exhibited. Preferably, the Mn content is 0.70% or more. On the other hand, if the Mn content is over 1.50%, the effect of addition becomes saturated, the time for completion of transformation at the time of isothermal transformation of the steel wire rod becomes longer, and the manufacturability deteriorates. The preferable Mn content is 1.30% or less.

P: 0.030% or Less

P is an element segregating at the crystal grain boundaries thereby causing deterioration of the cold formability. If the P content is over 0.030%, the deterioration of the cold formability becomes remarkable. The preferable P content is 0.015% or less. The steel wire rod, drawn steel wire, and parts according to the present embodiment do not have to contain P, so the lower limit of the P content is 0%.

S: 0.030% or Less

S, like P, is an element segregating at the crystal grain boundaries thereby causing deterioration of the cold formability. If the S content is over 0.030%, the deterioration of the cold formability becomes remarkable. The preferable S content is 0.015% or less, while the more preferable one is 0.010% or less. The steel wire rod, drawn steel wire, and parts according to the present embodiment do not have to contain S, so the lower limit of the S content is 0%.

Al: 0.005 to 0.060%

Al is a deoxidizing element. Further, it is an element which forms AlN functioning as pinning particles. AlN refines the crystal grains and due to this raises the cold formability. Further, Al is an element having the action of reducing the solute N to suppress dynamic strain aging. If the Al content is less than 0.005%, the above-mentioned effect cannot be obtained. The preferable Al content is 0.020% or more. If the Al content is over 0.060%, the above-mentioned effect becomes saturated and defects easily form at the time of hot rolling. The preferable Al content is 0.050% or less.

Ti: 0.005 to 0.030%

Ti is a deoxidizing element. Further, it is an element which forms TiN and has the action of reducing the solute N to suppress dynamic strain aging. If the Ti content is less than 0.005%, the above-mentioned effect cannot be obtained. The preferable Ti content is 0.010% or more. If the Ti content is over 0.030%, the above-mentioned effect becomes saturated and defects easily form at the time of hot rolling. The preferable Ti content is 0.025% or less.

B: 0.0003 to 0.0050%

B suppresses intergranular ferrite and has the effect of improving cold formability and the effect of promoting bainite transformation and improving strength. If less than 0.0003%, the effect is not sufficient, while if over 0.0050%, the effect becomes saturated.

N: 0.0010 to 0.0100%

N is an element sometimes causing cold formability to deteriorate due to dynamic strain aging. To avoid such a detrimental effect, the N content is made 0.0100% or less. Further, N forms AlN or TiN to refine the crystal grain size and has the effect of improving the cold formability. For this reason, the lower limit was made 0.0010%. The preferable content of N is 0.0020 to 0.0040%.

In the present invention, one or two of Cr: 0.01 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10% may be included. The inclusion of Cr, Nb, and V is optional. 0% is also possible. Cr has the effect of increasing the tensile strength of the steel. Nb and V have the effect of reducing the solute N to suppress dynamic strain aging and the effect of promoting bainite transformation to increase the strength.

Cr: 0.01 to 0.40%

Cr is an element increasing the tensile strength of the steel. If the Cr content is less than 0.01%, the above-mentioned effect is not sufficiently obtained. On the other hand, if the Cr content is over 0.40%, martensite easily forms. Due to this, the drawability and the cold forgeability deteriorate. The preferable content of Cr is 0.03 to 0.30%.

Nb: 0 to 0.03%

Nb is an element which forms NbN and has the action of reducing the solute N to suppress dynamic strain aging. If the Nb content is over 0.03%, the above-mentioned effect becomes saturated and defects easily form at the time of hot rolling. The Nb content is preferably 0.025% or less.

V: 0 to 0.10%

V is an element which forms VN and has the action of reducing the solute N to suppress dynamic strain aging. If the V content is over 0.10%, the above-mentioned effect becomes saturated and defects easily form at the time of hot rolling. The preferable V content is 0.05% or less.

O: 0 to 0.0030% or Less

O is present as an oxide of Al and Ti etc. in the steel wire rod, drawn steel wire, and parts (for example, machine parts). If the O content is over 0.0030%, coarse oxides are formed in the steel resulting in fatigue fracture easily occurring. The preferable O content is 0.0020% or less. The lower limit value of the O content is 0%.

Above, the chemical composition of the steel wire rod, drawn steel wire, and parts according to the present embodiment were explained, but the balance of the chemical composition consists of Fe and unavoidable impurities. Here, “unavoidable impurities” mean constituents contained in the raw materials or entering in the manufacturing process and not intentionally included in the steel. Further, “unavoidable impurities” specifically include for example Sb, Sn, W, Co, As, Mg, Pb, Bi, and H. Note that, Sb, Sn, W, Co, As, Mg, Pb, Bi, and H respectively are allowed to be contained in amounts of up to 0.010%, 0.10%, 0.50%, 0.50%, 0.010%, 0.010%, 0.10%, 0.10%, and 0.0010% for realizing the effects of the present application.

Next, the microstructure of the steel wire rod and drawn steel wire and the parts according to the present embodiment will be explained. The drawn steel wire according to the present embodiment is obtained by drawing the steel wire rod according to the present embodiment. The parts according to the present embodiment are obtained by cold forging or by cold forging and rolling the drawn steel wire according to the present embodiment. The effect of cold forging and rolling on the microstructure of the parts is small. This is because the amount of work on the parts by the cold forging and rolling is small.

Area Ratio of Bainite: 90% or More

The effect of drawing, cold forging, and rolling on the bainite area ratio of the microstructure is small, so the following explanation applies to all of the steel wire rod, drawn steel wire, and parts. The microstructure of the steel wire rod, drawn steel wire, and parts according to the present embodiment includes an area ratio of 90% or more of bainite. In the present embodiment, “bainite”, as shown in FIG. 1, refers to a structure in which it is deemed needle-shaped or particle-shaped cementite is dispersed if etching a cross section of an object (steel wire rod, drawn steel wire, or parts) (cross-section perpendicular to axis of steel material (drawn steel wire)) by Nital, then photographing a position of a predetermined depth (for example, depth from surface layer of 0.25 time diameter) from the surface layer of the object by a scan type electron microscope (SEM).

In the present embodiment, the bainite area ratio of steel wire rod, drawn steel wire, and parts is determined by the following procedure. That is, first, the cross section of the object is etched by Nital to bring out the structure. Next, if the diameter of the object is D, a total of nine locations are specified: four locations determined by rotation every 90° about the longitudinal direction axis of the object at a depth position of a depth from the surface layer of the object of 50 μm, four locations determined by rotation every 90° about the above axis at a depth position of a depth from the surface layer of the object of 0.25D, and one location determined at the center part of the above axis (depth position of a depth from the surface layer of 0.5D). Further, these nine locations are photographed for structure by a power of 1000× using an SEM. Furthermore, the nonbainite structures in the structural photographs (ferrite, pearlite, and martensite structures) are visually marked and the regions of the structures are found by image analysis. As a result, the region containing the bainite is found by subtracting the nonbainite regions from the overall field of observation. The area ratio of this region is made the area ratio of the bainite. Note that, this operation measures and calculates the values for at least two samples, finds the mean value of the same, and uses the mean value as the bainite area ratio in the present embodiment.

However, sometimes bainite is difficult to distinguish from a structural photograph obtained by an SEM. In this case, it is distinguished using an electron backscatter diffraction (EBSD) apparatus by the KAM method (Kernel Average Misorientation). The KAM method is the method of performing a calculation averaging the difference in orientation between pixels in first approximation of six adjoining pixels of a regular hexagonal shape in the measurement data, second approximation of the 12 at the outside of that, or third approximation of the 18 at the further outside of that and using this value as the value of the center pixel for the different pixels. This calculation is performed so that the grain boundaries are not exceeded and thereby a map expressing a change in orientation in the particles can be prepared. Bainite is larger is dislocation density and larger in strain in the particles compared with the polygonal pro-eutectoid ferrite transformed at a high temperature, so the difference in particles in crystal orientation is large. Therefore, in the analysis in the present embodiment, the conditions for calculation of the difference in orientation between adjoining pixels are made third approximation, particles with that difference in orientation of 5° or less are displayed, and particles with a difference in orientation among these of over 1° are defined as bainite.

Assuming such a method of discrimination of bainite, in the present embodiment, if the area ratio of bainite of the steel wire rod is less than 90%, the area ratio of bainite of the drawn steel wire obtained by drawing this steel wire rod and parts obtained by cold forging the drawn steel wire becomes less than 90%. In this case, the yield ratio (=0.2% proof stress/tensile strength) of the parts falls. For example, the permanent elongation at the time of use as a machine part deteriorates. In addition to bainite, pearlite, pro-eutectoid ferrite, martensite, etc. are sometimes contained in the drawn steel wire, but so long as the area ratio of bainite in the drawn steel wire is 90% or more, inclusion of microstructures other than bainite is allowed. Note that, if the area ratio of bainite of the drawn steel wire is less than 90%, the strength of the drawn steel wire (tensile strength and hardness etc.) becomes uneven, so the drawn steel wire easily cracks when cold working it into a part. Note that, drawn steel wire preferably does not contain any microstructures other than bainite, so the upper limit value of the area ratio of bainite of the drawn steel wire is 100%.

Mean bainite block size of Steel Wire Rod of 15 μm or Less

In the steel wire rod according to the present embodiment, the mean block size of the bainite measured at the cross section is 15 μm or less. Here, the “cross section” means the plane perpendicular to the axial direction of the steel wire rod. If the mean bainite block size measured at the cross section of the steel wire rod is over 15 μm, the ductility of the drawn drawn steel wire falls and due to this the cold formability of the drawn steel wire falls. Furthermore, the mean block size of the bainite of the part obtained by cold working this drawn steel wire becomes coarse. If the mean block size of the bainite becomes coarser, the yield ratio falls. Note that, the mean block size of the bainite of the steel wire rod is preferably smaller, so there is no need to prescribe the lower limit value.

Mean Aspect Ratio R of Block Particles of Bainite of Drawn Steel Wire and Parts of 1.2 to 2.0

In the drawn steel wire and parts according to the present embodiment, at the position of the surface layer of the drawn steel wire, the mean aspect ratio R of the block particles of bainite measured at the longitudinal section of the drawn steel wire is 1.2 to 2.0. Here, the “longitudinal section” means the plane parallel to the axial direction of the steel wire rod and including the center axis. If the mean aspect ratio of the bainite block is less than 1.2, the hydrogen embrittlement resistance of parts manufactured by cold forging the drawn steel wire deteriorates. Further, if the mean aspect ratio exceeds 2.0, the yield ratio falls and the permanent elongation when used as parts deteriorates.

In the present embodiment, the mean aspect ratio R of the block particles of bainite of the drawn steel wire and parts is determined as follows. First, the bainite block grain boundary is determined using EBSD for the longitudinal section of the drawn steel wire. At this time, in the two regions from the surfaces of the two sides of the longitudinal section by 100 μm in the direction of the center axis of the drawn steel wire and by 500 μm in the direction of the center axis of the drawn steel wire, the crystal orientations of bcc-Fe at the measurement points in the regions are measured in measurement steps of 0.3 μm and a boundary with a difference in orientation of 15 degrees or more is defined as a “bainite block boundary”. Further, the region surrounded by this boundary is defined as a “bainite block grain”. In this way, a map of bainite block grains at a total of two regions at the two sides of one longitudinal section is obtained. This is performed for four samples to obtain a map of bainite block grains at a total of eight regions. 10 bainite block grains are selected in order from the one with the largest circle equivalent diameter from the map of bainite block grains obtained. The selected 10 bainite block grains are measured for aspect ratio of the block grains. Finally, the mean value of these is made the mean aspect ratio R of the block grains of the bainite.

Mean bainite block size of drawn steel wire of (15/R) μm or Less

In the drawn steel wire according to the present embodiment, the mean block size of the bainite of the surface layer measured at the cross section is (15/R) μm or less. Here, the “cross section” means the plane perpendicular to the axial direction of the drawn steel wire. If the mean bainite block size of the surface layer of the drawn steel wire measured at the cross section is over (15/R) μm, the ductility of the drawn steel wire falls and due to this the cold formability of the drawn steel wire falls. Furthermore, the mean block size of the bainite of the parts obtained by cold working this drawn steel wire becomes coarser and the yield strength falls. Note that, the mean block size of the bainite at the surface layer part of the drawn steel wire is preferably small, so the lower limit value does not have to be prescribed.

In the present embodiment, the mean bainite block size at the surface layer of the steel wire rod (similar for drawn steel wire and parts) is determined as follows: First, at the cross section of the steel wire rod, the region extending 500 μm in the peripheral direction from the surface layer in the center axial direction by 500 μm width is determined. Four regions rotated every 90° about the center axis in this region are specified. Further, the block particle sizes of these four regions measured by an EBSD apparatus are averaged to obtain the mean bainite block size at the surface layer of the steel wire rod (similar for drawn steel wire and parts).

Mean Bainite Block Size of Parts of (15/R) μm or Less

In the parts according to the present embodiment, the mean bainite block size of the surface layer measured at the cross section is (15/R) μm or less. Here, the “cross section” means the plane perpendicular to the axial direction of the parts. If the mean bainite block size of the surface layer of the parts measured at the cross section is over (15/R) μm, the yield ratio falls. Note that, the mean bainite block size of the surface layer of the drawn steel wire is preferably small, so there is no need to prescribe the lower limit value. Further, the method of determination of the mean bainite block size of the parts is the same as the above-mentioned method of determination of the mean bainite block size of the steel wire rod.

(Mean Bainite Block Size of Surface Layer of Steel Wire Rod, Drawn Steel Wire, and Parts)/(Mean Bainite Block Size at Center Part) of Less than 1.0

In the steel wire rod, drawn steel wire, and parts according to the present embodiment, the ratio of the mean bainite block size of the surface layer measured at the cross section and the mean bainite block size of the center part measured at the cross section is less than 1.0. If the ratio is over 1.0, the cold forgeability of the drawn steel wire deteriorates and the yield ratio of the parts deteriorates.

In the present embodiment, the mean block size of the bainite at the center part of the steel wire rod (similar for drawn steel wire and parts) is determined as follows: First, in the cross section of the steel wire rod, a region of 500 μm×500 μm centered about the center axis is determined. The block particle size of this region is measured by an EBSD apparatus. Next, three different cross sections are similarly measured, then the block particle sizes of the four samples are averaged to obtain the mean block size of the bainite at the center part of the steel wire rod (similar for drawn steel wire and parts).

Further, in the present embodiment, the ratio of the block particle size of the surface layer and the block particle size of the center part is found by (mean bainite block size of surface layer)/(mean bainite block size at center part).

Mean Particle Size of Cementite Dispersed in Bainite of 0.1 μm or Less

In the steel wire rod, drawn steel wire, and parts according to the present embodiment, the mean particle size of cementite dispersed in the bainite is 0.1 μm or less. If the mean particle size of the cementite exceeds 0.1 μm, the cold forgeability of the drawn steel wire deteriorates. Furthermore, the yield ratio of the parts fall and the permanent elongation when used as for example a machine part deteriorates.

The mean particle size of cementite in the bainite according to the present embodiment is determined by the following procedure. First, picral is used to etch the cross section of the object (steel wire rod, drawn steel wire, or parts) to bring out the structure. Next, if the diameter of the object is D, a total of nine locations are specified: four locations determined by rotation every 90° about the longitudinal direction axis of the object at a depth position of a depth from the surface layer of the object of 50 μm, four locations determined by rotation every 90° about the above axis at a depth position of a depth from the surface layer of the object of 0.25D, and one location determined at the center part of the above axis (depth position of a depth from the surface layer of 0.5D). Further, these nine locations are photographed for structure by a power of 20000× using a field emission scan type electron microscope (FE-SEM). Finally, the captured images are binarized, circle equivalent diameters of cementite are found by image analysis, and the mean value of the nine samples is calculated to obtain the mean particle size of the cementite.

Critical Upsetting Rate of Drawn Steel Wire of 80% or More

The drawn steel wire obtained in the above way exhibits excellent cold formability. In the present embodiment, the critical upsetting rate is used as an indicator showing the cold formability. In the present embodiment, the “critical upsetting rate” means the maximum compression rate where no cracking occurs when preparing a sample with a height of 1.5 times the diameter from drawn steel wire by machining and compressing the end face of the sample in the axial direction using a die formed with grooves concentrically. Note that, the “compression rate” is the value shown by ((H−H1)/H)×100 where the height before compression in the drawing (axial direction dimension) is “H” and the height after compression in the drawing (axial direction dimension) is “H1”. In the drawn steel wire according to the present embodiment, the critical upsetting rate can be made 80% or more and an excellent cold formability can be realized.

Next, one example of the methods of production of the steel wire rod, drawn steel wire, and parts will be explained. First, a steel slab having a chemical composition containing, by mass %, C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, and N: 0.001 to 0.010%, containing, in accordance with need, one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10%, and having a balance of Fe and impurities is prepared. This steel slab is heated to 1000 to 1150° C., then hot rolled by a finish rolling temperature of 800 to 950° C. to thereby obtain a steel wire rod. Next, this 800 to 950° C. steel wire rod is cooled by a mean cooling rate of 40° C./s or more down to 600° C., then is cooled by a mean cooling rate of 25° C./s or more down to 480° C. After that, this steel wire rod is held thermostatically in a 400 to 480° C. temperature zone for 15 seconds or more (first isothermal holding) and further is held thermostatically in a 530 to 600° C. temperature zone for 25 seconds or more (second isothermal holding). Further, after that, the material is water cooled to obtain a steel wire rod.

The two-stage cooling after finish rolling and the first isothermal holding are performed by immersing the steel wire rod in 400 to 480° C. molten salt inside a first molten salt tank. Further, the second isothermal holding is performed by immersing the steel wire rod in 530 to 600° C. molten salt inside a second molten salt tank.

Here, in the method of production of steel wire rod according to the present embodiment, in particular, the cooling of the steel wire rod from 800 to 950° C. is divided into two stages of cooling down to 600° C. and cooling from 600° C. down to 480° C. In particular, in the latter cooling, the cooling rate can be made 25° C./s or more to thereby control the mean block size of the bainite to 15 μm or less.

Further, in the method of production of steel wire rod according to the present embodiment, the molten salt bath temperature in the first molten salt tank is made 400 to 480° C. and the immersion time is made 15 to 50 seconds. By making the molten salt bath temperature 400° C. or more, the entry of martensite is suppressed and excellent cold forgeability is obtained. On the other hand, by making it 480° C. or less, it is possible to reduce the mean particle size of the cementite to obtain excellent cold forgeability and eliminate bluing. Further, by making the immersion time 15 seconds or more, the entry of nonbainite structures is suppressed and excellent cold forgeability is obtained. On the other hand, by making it 50 seconds or less, it is possible to reduce the mean particle size of the cementite to obtain excellent cold forgeability and eliminate bluing.

Similarly, in the method of production of steel wire rod according to the present embodiment, the molten salt bath temperature in the second molten salt tank is made 530 to 600° C. and the immersion time is made 25 to 80 seconds. By making the molten salt bath temperature 530° C. or more, the entry of martensite is suppressed and excellent cold forgeability is obtained. On the other hand, by making it 600° C. or less, it is possible to reduce the mean particle size of the cementite to obtain excellent cold forgeability and eliminate bluing. Further, by making the immersion time 25 seconds or more, the entry of martensite is suppressed and excellent cold forgeability is obtained. On the other hand, by making it 80 seconds or less, it is possible to reduce the mean particle size of the cementite to obtain excellent cold forgeability and eliminate bluing.

Next, the drawn steel wire according to the present embodiment can as one example be manufactured by the following method. That is, the steel wire rod manufactured by the above-mentioned method is drawn by a total area reduction rates of 10 to 55%. The total area reduction rate 10 to 55% in the drawing operation may be reached by a single drawing operation or may be realized by a plurality of drawing operations. The drawn steel wire according to the present embodiment is obtained in this way.

Further, the parts of the present embodiment (machine parts, building parts, etc.) can as one example be manufactured by the following method. That is, the above-mentioned drawn steel wire is worked into the shape of the individual parts by cold forging or cold forging and rolling to obtain parts with a tensile strength of 700 to 1200 MPa.

EXAMPLES

Next, examples of the present invention will be explained, but the conditions in the examples are an illustration of conditions employed for confirming the workability and effects of the present invention. The present invention is not limited to this illustration of conditions. The present invention can employ various conditions so long as not departing from the gist of the present invention and achieving the object of the present invention.

Steel slabs of the 14 types of chemical composition shown in Table 1 were successively heated, hot rolled, isothermally transformed, and cooled under the 28 patterns of conditions shown in Table 2 to produce steel wire rods (Levels 1 to 28). Next, the steel wire rods were drawn by the area reduction rates shown in Table 2 to produce drawn steel wire s (Levels 1 to 28). Furthermore, the drawn steel wires were used to prepare samples with heights of 1.5 times the diameters by machining to produce parts (Levels 1 to 28). Further, the end faces of the parts were compressed in the axial direction using dies with concentric grooves and the maximum compression rates where no cracking occurred were defined as the critical upsetting rates of the parts. Further, drawn steel wires with critical upsetting rates of 80% or more were judged as excellent in cold formability. Further, tensile test pieces were sampled from the shaft parts of the parts, subjected to tensile tests, and measured for tensile strength and 0.2% proof stress. Parts with a yield ratio (0.2% proof stress/tensile strength) of 0.90 or more were judged excellent in yield ratio. Note that, for each of steel materials, drawn steel wires, and parts, the Levels 1 to 7 and Levels 14 to 20 are invention examples while the Levels 8 to 13 and Levels 21 to 28 are comparative examples.

TABLE 1 Steel type C Si Mn P S Al Ti Cr B N Nb V o A 0.17 0.22 1.45 0.011 0.009 0.032 0.017 0.22 0.0018 0.0035 — — 0.0019 B 0.19 0.07 0.98 0.009 0.007 0.035 0.021 0.19 0.0021 0.0028 0.015 — 0.0016 C 0.21 0.18 1.07 0.014 0.011 0.055 0.00

0.21 0.0020 0.0039 — 0.06 0.0017 D 0.22 0.21 1.22 0.012 0.009 0.010 0.023 0.23 0.0017 0.0033 — — 0.0011 E 0.23 0.19 1.23 0.008 0.013 0.029 0.019 0.14 0.0014 0.0040 — — 0.0010 F 0.25 0.22 1.05 0.010 0.014 0.031 0.01

0.02 0.0023 0.003

— — 0.000

G 0.28 0.14 0.99 0.009 0.012 0.041 0.011 0.25 0.0009 0.0032 — — 0.0010 H 0.16 0.36 1.48 0.013 0.01

0.00

0.024 — 0.0008 0.002

0.011 0.04 0.0017 I 0.19 0.25 1.26 0.009 0.010 0.038 0.01

— 0.0013 0.0032 0.00

— 0.0015 J 0.22 0.21 1.07 0.011 0.007 0.04

0.012 — 0.0017 0.0027 — 0.02 0.0012 K 0.23 0.15 1.42 0.012 0.011 0.037 0.014 — 0.0024 0.0038 — — 0.001

L 0.25 0.24 1.06 0.010 0.009 0.029 0.007 — 0.0019 0.0021 — — 0.0014 M 0.28 0.08 1.45 0.008 0.009 0.033 0.022 — 0.0016 0.0037 — — 0.000

N 0.2

0.13 0.9

0.009 0.008 0.027 0.017 — 0.0011 0.003

— — 0.0011

indicates data missing or illegible when filed

TABLE 2 First molten Second molten Mean cooling Mean cooling salt tank salt Total Finish rate down Mean cooling rate down Immer- Immer- area Wire Heating rolling to 600° C. rate 600° C. to

00° C. Bath sion Bath sion reduction Steel size temp. temp. after rolling to 480° C. after rolling temp. time temp. time rate Level type (mm) (° C.) (° C.) (° C./s) (° C./s) (° C./s) (° C.) (

) (° C.) (

) (%) Remarks 1 A 12.0 1070 880

1 35 45 480 25 540 40 30.

Inv. ex. 2 B 12.0 1080 880 47 33 4

450 25 540 40 30.

Inv. ex. 3 C 12.0 1080 850 52 35 48 4

0 25 5

0 40 30.

Inv. ex. 4 D 12.0 1080 8

0 45 3

40 450 25 550 40 30.

Inv. ex. 5 E 12.0 1070 8

0 52 3

48 450 25 550 40 30.

Inv. ex. 6 F 12.0 1100 8

0 44 32 3

450 25 550 40 30.

Inv. ex. 7 G 12.0 1100 880 48 33 43 480 25 540 40 30.

Inv. ex. 8 A 12.0 10

0 880 40 — 35 4

0 25

50 40 30.

Comp. ex. 9 C 12.0 10

0 880 47 32 41 450 25 700

00 30.

Comp. ex. 10 E 12.0 10

0 880 11 11 11 — — — — 30.

Comp. ex. 11 F 12.0 1080 8

0 1.2   1.2   1.2 — — — — 30.

Comp. ex. 12 D 12.0 1100 8

0 42 — 3

50

25 570 40 30.

Comp. ex. 13 F 12.0 10

0 8

0 — — — 440 90 — — 30.

Comp. ex. 14 H 12.5 1100 8

0 47 30 41 450 25 550 40 3

.0 Inv. ex. 15 I 12.5 1080 8

0 49 31 44 450 25 550 40 3

.0 Inv. ex. 16 J 12.5 1080 880 49 28 43 4

0 2

540 40 3

.0 Inv. ex. 17 K 12.5 1110

0

45 30 40 450 2

550 40 3

.0 Inv. ex. 18 L 12.5 1080

00 48 28 41 4

0 2

540 40 3

.0 Inv. ex. 19 M 12.5 1100 880 48

1 43 450 2

5

0 40 3

.0 Inv. ex. 20 N 12.5 1080

00 47 28 42 450 25 5

0 40 3

.0 Inv. ex. 21 K 12.5 10

0 8

0 45 — 41 4

0

3

50 10

38.0 Comp. ex. 22 M 18.0 1100

00 32 21 2

450 25 700

00 30.

Comp. ex. 23 K 12.5 10

0 8

0 51 35 48 350 25 540 40 3

.0 Comp. ex. 24 K 12.5 10

0 8

0 43 — — 450 10

50 30 38.0 Comp. ex. 25 M 12.5 10

0 8

0 45 30 42 450

0 550 75 38.0 Comp. ex. 26 K 12.5 10

0 8

0 47 31 43 450 25 480 40 35.0 Comp. ex. 27 M 12.5 10

0 8

0 48 31 42 450 25 550 10 3

.0 Comp. ex. 28 M 12.5 1100

0

47 32 42 450 25 550 1

0 3

.0 Comp. ex.

indicates data missing or illegible when filed

Note that, if explaining the levels including the blank fields of Table 2, for example, Level 10 is an example of not performing isothermal transformation after hot rolling, but immersing the sample in a boiling water tank for production. Level 11 is an example of not performing isothermal transformation after hot rolling, but cooling by cooling air for production. Level 13 is an example of cooling the hot rolled steel wire rod once down to room temperature, then reheating it to 1000° C. and immersing it in one molten salt tank for production.

Next, Table 3 shows the results relating to the structure of the steel wire rod, Table 4 shows the results relating to the structure of the drawn steel wire, and Table 5 shows the results for the cold forgeability of the drawn steel wire and the characteristics of the parts.

TABLE 3 Structure of wire rods Ratio of Mean Area Mean block mean block particle ratio of particle size particle sizes size of bainite at surface layer at surface layer cementite Level (%) (μm) and center (μm) Remarks 1 92 13.6 0.93 0.055 Inv. ex. 2 92 14.2 0.94 0.047 Inv. ex. 3 95 14.5 0.92 0.062 Inv. ex. 4 94 13.2 0.94 0.060 Inv. ex. 5 98 14.1 0.91 0.064 Inv. ex. 6 96 13.1 0.93 0.063 Inv. ex. 7 93 14.5 0.92 0.071 Inv. ex. 8 95 14.8 0.94 0.22 Comp. ex. 9 93 13.9 0.94 0.56 Comp. ex. 10 80 14.2 0.92 0.30 Comp. ex. 11 22 21.9 0.93 1.7 Comp. ex. 12 92 15.5 0.92 0.083 Comp. ex. 13 97 14.7 1.12 0.061 Comp. ex. 14 91 14.1 0.92 0.052 Inv. ex. 15 92 13.8 0.93 0.058 Inv. ex. 16 93 14.2 0.91 0.054 Inv. ex. 17 94 14.0 0.92 0.062 Inv. ex. 18 92 13.7 0.92 0.059 Inv. ex. 19 93 13.9 0.91 0.063 Inv. ex. 20 94 13.8 0.93 0.066 Inv. ex. 21 93 14.2 0.93 0.24 Comp. ex. 22 92 14.5 0.92 0.61 Comp. ex. 23 72 13.3 0.91 0.051 Comp. ex. 24 81 14.1 0.93 0.082 Comp. ex. 25 95 14.2 0.92 0.13 Comp. ex. 26 84 13.9 0.93 0.059 Comp. ex. 27 85 13.8 0.92 0.052 Comp. ex. 28 94 14.1 0.92 0.15 Comp. ex.

TABLE 4 Structure of steel wire Mean Upper limit Ratio of mean particle Area ratio Mean aspect of mean block Mean block block particle size of of bainite ratio of block particle size particle size sizes at surface cementite Level (%) particles (μm) (μm) layer and center (μm) Remarks 1 92 1.6 9.4 8.2 0.91 0.051 Inv. ex. 2

3 1.4 10.7

.4 0.

2 0.045 Inv. ex. 3 94 1.5 10.0 9.1 0.91 0.062 Inv. ex. 4 95 1.7 8.8 7.9 0.93 0.058 Inv. ex. 5 97 1.5 10.0 9.2 0.90 0.061 Inv. ex. 6 96 1.6 9.4 8.1 0.91 0.062 Inv. ex. 7 94 1.4 10.7 9.5 0.92 0.067 Inv. ex. 8 94 1.5 10.0 9.2 0.93 0.21 Comp. ex. 9 92 1.6 9.4 8.3 0.92 0.51 Comp. ex. 10 81 1.5 10.0 8.6 0.92 0.27 Comp. ex. 11 23 1.4 10.7 12.7  0.92 1.7 Comp. ex. 12 92 1.6 9.4 9.7 0.91 0.082 Comp. ex. 13 97 1.5 10.0 9.7 1.1 0.06 Comp. ex. 14 92 1.8 8.3 7.3 0.90 0.048 Inv. ex. 15

2 1.7 8.8 7.2 0.81 0.055 Inv. ex. 16

2 1.8 8.3 7.1 0.90 0.052 Inv. ex. 17 83 1.

7.9 6.9 0.91 0.060 Inv. ex. 18

2 1.8 8.3 7.0 0.90 0.058 Inv. ex. 19

4 1.7 8.8 7.7 0.90 0.061 Inv. ex. 20

5 1.

7.9 6.9 0.91 0.065 Inv. ex. 21

3 1.8 8.3 7.0 0.92 0.23 Comp. ex. 22

3 1.7 8.8 8.0 0.92 0.56 Comp. ex. 23 71 1.

9.4 7.

0.90 0.050 Comp. ex. 24 80 1.7 8.8 8.1 0.92 0.081 Comp. ex. 25 96 1.9 7.9 7.2 0.91 0.13 Comp. ex. 26 85 1.9 7.9 7.1 0.90 0.058 Comp. ex. 27 85 2.0 7.5 6.

0.92 0.051 Comp. ex. 28 95 1.9 7.9 7.3 0.90 0.15 Comp. ex.

indicates data missing or illegible when filed

TABLE 5 Cold forgeability of drawn steel wire Characteristics of parts Limit compression Tensile strength Yield strength Level rate Evaluation (MPa) ratio Remarks 1 80% or more Good 823 0.95 Inv. ex. 2 80% or more Good 854 0.94 Inv. ex. 3 80% or more Good 896 0.95 Inv. ex. 4 80% or more Good 925 0.94 Inv. ex. 5 80% or more Good 935 0.92 Inv. ex. 6 80% or more Good 947 0.94 Inv. ex. 7 80% or more Good 965 0.93 Inv. ex. 8 77% No good 776 0.90 Comp. ex. 9 74% No good 748 0.91 Comp. ex. 10 76% No good 892 0.82 Comp. ex. 11 74% No good 765 0.79 Comp. ex. 12 76% No good 912 0.91 Comp. ex. 13 77% No good 961 0.90 Comp. ex. 14 80% or more Good 811 0.94 Inv. ex. 15 80% or more Good 862 0.93 Inv. ex. 16 80% or more Good 942 0.93 Inv. ex. 17 80% or more Good 953 0.92 Inv. ex. 18 80% or more Good 962 0.94 Inv. ex. 19 80% or more Good 996 0.93 Inv. ex. 20 80% or more Good 989 0.92 Inv. ex. 21 75% No good 883 0.89 Comp. ex. 22 73% No good 789 0.83 Comp. ex. 23 71% No good 1053 0.88 Comp. ex. 24 72% No good 978 0.82 Comp. ex. 25 80% or more Good 981 0.87 Comp. ex. 26 73% No good 971 0.88 Comp. ex. 27 71% No good 1022 0.85 Comp. ex. 28 80% or more Good 975 0.86 Comp. ex.

As clear from Tables 2 to 5, regarding Levels 1 to 7 and Levels 14 to 20 (invention examples) where all of the manufacturing conditions prescribed in the present application are inside the prescribed ranges, in each case, a good result is obtained for the cold forgeability of the drawn steel wire and characteristics of the parts. That is, regarding Levels 1 to 7 and Levels 14 to 20, it will be understood that in each case, the tensile strength of the parts is 700 to 1200 MPa and that a yield ratio of 0.90 or more is obtained even without so-called bluing after forming the parts.

As opposed to this, regarding Levels 8 to 13 and Levels 21 to 28 (comparative examples) where at least one of the manufacturing conditions prescribed in the present application is outside the prescribed range, it will be understood that the result shows that at least one of the cold forgeability of the drawn steel wire and characteristics of the part is not excellent.

INDUSTRIAL APPLICABILITY

As shown above, according to the present invention, parts with a tensile strength of 700 to 1200 MPa which can be inexpensively manufactured are obtained. Further, drawn steel wire used for manufacture of the parts and enabling elimination of spheroidizing annealing, quenching and tempering, and bluing after cold forging and steel wire rod for manufacturing that drawn steel wire can be obtained. Therefore, the present invention has high applicability in the field of manufacturing of steel members, so is promising. 

1. Steel wire rod comprised of, by mass %, C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, and N: 0.001 to 0.010% and having a balance of Fe and unavoidable impurities, in which steel wire rod, 90% or more of the microstructure by area ratio is bainite, a mean bainite block size of the surface layer measured at the cross section is 15 μm or less, a value of a ratio of the mean bainite block size of the surface layer measured at the cross-section and the mean bainite block size measured at the center part (mean bainite block size of surface layer)/(mean bainite block size at center part) is less than 1.0, and a mean particle size of cementite dispersed in bainite is 0.1 μm or less.
 2. The steel wire rod according to claim 1, wherein said steel wire rod further comprises, by mass %, one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10%.
 3. Drawn steel wire comprised of, by mass %, C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, and N: 0.001 to 0.010% and having a balance of Fe and unavoidable impurities, in which drawn steel wire, 90% or more of the microstructure by area ratio is bainite, at the surface layer of the drawn steel wire, a mean aspect ratio R of block particles of bainite measured at the longitudinal section is 1.2 to 2.0, a mean bainite block size of the surface layer measured at the cross section is (15/R)μm or less, a value of a ratio of the mean bainite block size of the surface layer measured at the cross-section and the mean bainite block size measured at the center part (mean bainite block size of surface layer)/(mean bainite block size at center part) is less than 1.0, and a mean particle size of cementite dispersed in bainite is 0.1 μm or less.
 4. The drawn steel wire according to claim 3, wherein said drawn steel wire further comprises, by mass %, one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10%.
 5. The drawn steel wire according to claim 3, wherein a critical upsetting rate is 80% or more.
 6. A part comprised of, by mass %, C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, and N: 0.001 to 0.010% and having a balance of Fe and unavoidable impurities, in which part, 90% or more of the microstructure by area ratio is bainite, at a surface layer of the part, a mean aspect ratio R of block particles of bainite measured at the longitudinal section is 1.2 to 2.0, a mean bainite block size of the surface layer measured at the cross-section is (15/R) μm or less, a value of a ratio of the mean bainite block size of the surface layer measured at the cross-section and the mean bainite block size measured at the center part (mean bainite block size of surface layer)/(mean bainite block size at center part) is less than 1.0, and a mean particle size of cementite dispersed in bainite is 0.1 μm or less.
 7. The part according to claim 6, wherein said part further comprises, by mass %, one or two of Cr: 0 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10%.
 8. The drawn steel wire according to claim 4, wherein a critical upsetting rate is 80% or more. 